The 3rd-Generation Partnership Project (3GPP) specifications refer to end-user wireless communication devices as “User Equipment” (UEs). UEs are also known as mobile terminals, wireless terminals and/or mobile stations, and are configured to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular network. The communication may be performed, e.g., between two UEs, between a user equipment and a regular telephone and/or between a UE and a server, via a Radio Access Network (RAN) and possibly one or more core networks that together make up the cellular communications network.
Various examples of and/or alternative names for UEs include mobile telephones, cellular telephones, laptops, or table computers with wireless capability, to name a few examples. UEs in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as with another UE or with a server. The concept of “user equipment” also includes devices with communications capability of a machine-type character, such as wireless-enabled sensors, measurement devices, etc., where the device is not necessarily interacting with a human user at all.
A cellular communications network covers a geographical area that is divided into cell areas, where each cell area is served by a base station, e.g., a Radio Base Station (RBS). An RBS may sometimes be referred to as, e.g., “base station”, “eNodeB”, “NodeB”, “B node”, or “BTS” (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes, such as macro eNodeBs, home eNodeBs or pico base stations, where the classification is based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site.
LTE Mobility
Mobility management is a challenging task in cellular communications systems, and well-functioning mobility management is crucial to the quality experienced by the end user of the wireless system. In LTE systems, the Radio Resource Control protocol (RRC) is the main signaling protocol for configuring connections, re-configuring connections, and other general connection handling in the LTE radio access network (E-UTRAN). (See 3GPP TS 36.331, available at www.3gpp.org.) RRC controls many functions such as connection setup, mobility, measurements, radio link failure and connection recovery.
A user equipment (UE) in LTE can be in one of two RRC states at any given time: RRC_CONNECTED state and RRC_IDLE state. In RRC_CONNECTED state, the UE's mobility is controlled by the network, based on, for example, measurements provided by the user equipment. That is, the network decides when and to which cell a UE should be handed over, based on, for example, measurement reports provided by the user equipment. The network, e.g., the LTE radio base station (called an eNodeB or eNB) in E-UTRAN, configures various measurement events, thresholds, etc. Based on this configuration, the user equipment then sends measurement reports to the network, so that the network can make a decision to hand over the user equipment to a stronger cell as the user equipment moves away from the present cell.
FIG. 1 illustrates a simplified signaling scheme for the LTE handover (HO) procedure. It should be noted that in LTE systems the HO command is in fact prepared in the Target eNB, i.e., the eNodeB that the user equipment will be handed over to, but the message is transmitted via the Source eNB. That is, the message comes from the Source eNB, from the user equipment's perspective.
In RRC_IDLE state, mobility is handled by UE-based cell-reselection, where a nomadic user equipment selects the “best” cell to camp on, based on, for example, various specified criteria and parameters that are broadcasted in the cells. For example, various cells or frequency layers can be prioritized over other, such that the user equipment tries to camp on a particular cell as long as the measured quality of a beacon or pilot in that cell is better than some other beacon or pilot received from other cells by at least a threshold quantity.
The present disclosure is primarily focusing on problems associated with network-controlled mobility as described above, e.g., for an LTE user equipment in RRC_CONNECTED state. The problems associated with losing RRC connection are therefore described in further detail below.
In a regular situation, and when a RRC_CONNECTED user equipment is moving out from the coverage of a first cell, also called a source cell, it should be handed over to a neighboring cell, also called a target cell or second cell, before the connection to the first cell is lost. That is, it is desirable that the connection is maintained with no or minimal disruption throughout the handover, such that the end-user is unaware of the ongoing handover. In order to succeed with this, it is necessary that:                a measurement report that indicates the need for mobility is transmitted by the user equipment and received by the Source eNB (see FIG. 1, item 110), and        the Source eNB has sufficient time to prepare the handover to the target cell (by, among other things, requesting a handover from the Target eNB controlling the target cell; see FIG. 1, item 120), and        the user equipment receives a handover command message from the network, as prepared by the Target eNB in control of the target cell and sent via the source cell to the user equipment (see FIG. 1, item 130).        
In addition, and in order for the handover to be successful, the user equipment must finally succeed in establishing a connection to the target cell, which in LTE requires a successful random access request in the target cell (see FIG. 1, item 140) and a subsequent transmission of a HO complete message from the user equipment to the Target eNB (FIG. 1, item 150). It should be noted that specifications may differ somewhat in the naming of messages.
Thus, it is clear that in order for the handover to succeed, it is necessary that the sequence of events leading to a successful handover is started sufficiently early, so that the radio link to the first cell over which this signaling takes place does not deteriorate too much before completion of the signaling. If such deterioration happens before the handover signaling is completed in the source cell (i.e., the first cell), then the handover is likely to fail. Such handover failures are clearly not desirable. The current RRC specifications for LTE therefore provide various triggers, timers, and thresholds in order to adequately configure measurements, such that the need for handovers can be detected reliably, and sufficiently early.
In the process shown in FIG. 1, for example, the measurement report (item 110) is triggered by a measurement event called A3 event (item 105). This means that a measurement report is sent to the network when a criterion or criteria associated with the event is satisfied. As defined by the 3GPP specifications, an A3 event means, in short, that the signal from a neighbor cell is found to be better than the signal from the current serving cell, by at least a certain offset. There exist many different measurement event types, and there are multiple events that can be configured to trigger a report.
A network node in control of a cell, such as an eNodeB in LTE terminology, maintains a neighbor cell relation list. Whenever a reference is made to a neighbor cell in this disclosure, it should be understood as a reference to a cell that is typically included in the neighbor cell relation list of a network node. A neighbor cell is thus a cell that may be a candidate for a handover. In some cases, a network node maintains a separate neighbor cell list for each cell that it controls. From the perspective of the user equipment, a neighbor cell is a cell in the proximity of or overlapping with the cell to which the user equipment is currently connected.
Radio Link Failure and RRC Connection Re-Establishment
It may occur that a UE loses coverage from the cell that the user equipment is currently connected to. This could occur in a situation when the UE enters a fading dip, for example, or if a handover was needed as described above but the handover fails for one or another reason.
The quality of the radio link is typically monitored in the user equipment, e.g., on the physical layer, as described in the most recent versions of 3GPP TS 36.300, 3GPP TS 36.331, and 3GPP TS 36.133, and as summarized below. In this disclosure, “layer” refers to a protocol layer as implemented by a processing circuit executing appropriate firmware and/or software. Thus, a typical UE may comprise one or more processing circuits executing a protocol stack, such that the UE may be regarded as comprising several “layers,” such as the physical layer, a data link layer, a network layer, etc.
Upon detecting that the physical layer is experiencing problems, e.g., according to criteria defined in 3GPP TS 36.133, the physical layer sends, to the RRC protocol, an indication of the detected problems. This indication is referred to as an out-of-sync indication. After a configurable number N310 of such consecutive indications, a timer T310 is started. If the link quality is not improved (recovered) while timer T310 is running, i.e., there are no N311 consecutive “in-sync” indications from the physical layer, a radio link failure (RLF) is declared in the user equipment. This sequence of events is shown in FIG. 2.
The functions of the currently relevant timers and counters described above are listed in Table 1, for reference. The UE may read the timer values and counter constants from system information broadcasted in the cell. Alternatively, it is possible to configure the UE with UE-specific values of the timers and counter constants using dedicated signaling, i.e., where specific values and constants are given to a particular UE or group of UEs with messages directed only to that UE or group of UEs.
TABLE 1TimerStartStopAt expiryT310Upon detectingUpon receiving N311If security isphysical layerconsecutive in-syncnot activated:problems i.e.indications from lowergo toupon receivinglayers, upon triggeringRRC_IDLE else:N310 consecutivethe handover proce-initiate theout-of-syncdure and upon initiat-connection re-indicationsing the connection re-establishmentfrom lower layersestablishmentprocedureprocedureT311Upon initiatingSelection ofEnterthe RRC connectiona suitable E-UTRARRC_IDLEre-establishmentcell or a cellprocedureusing another RAT.ConstantUsageN310Maximum number of consecutive “out-of-sync”indications received from lower layersN311Maximum number of consecutive “in-sync”indications received from lower layers
If timer T310 expires, indicating that a radio-link failure (RLF) has occurred, then the UE initiates a connection re-establishment to recover the ongoing RRC connection. This procedure includes cell selection by the user equipment. That is, the RRC_CONNECTED UE shall autonomously try to find a better cell to connect to, since the connection to the previous cell failed according to the described measurements. It could occur that the UE returns to the first cell anyway, but the same procedure is executed in any event. Once a suitable cell is selected as further described, e.g., in 3GPP TS 36.304, the UE requests a re-establishment of the connection in the selected cell. It is important to note the difference in mobility behavior when an RLF results in UE-based cell selection, in contrast to the normally applied network-controlled mobility.
If the re-establishment is successful, which depends on, among other things, whether the eNB controlling the selected cell is prepared to maintain the connection to the UE, which implies that it is prepared to accept the re-establishment request, then the connection between the UE and the network can resume, through the newly selected eNB (or the re-selected eNB, if the connection is re-established to the same eNB). In LTE, a re-establishment procedure includes a random-access request in the selected cell, followed by higher layer signaling where the user equipment sends a message with content that be used to identify and authenticate the UE. This is needed so that the network can trust that it knows exactly which UE is attempting to perform the re-establishment.
If the re-establishment attempt fails, the UE goes to RRC_IDLE state and the connection is released. To continue communication, a new RRC connection must then be requested and established. A re-establishment failure could occur, for example, if the eNB that receives the re-establishment request is unable to identify the UE that requests the re-establishment. Such a condition may occur if the receiving eNB has not been informed or otherwise prepared for a possible re-establishment from this UE.
The reason for introducing the timers T31x and counters N31x described above is to add some freedom and hysteresis for configuring the criteria for when a radio link should be considered as failed and needing to be re-established. This flexibility is desirable, since it would affect the end-user performance negatively if a connection is abandoned prematurely if it turned out that the loss of link quality was temporary and the UE succeeded in recovering the connection without any further actions or procedures, e.g., before T310 expires or before the counter reaches value N310.
The re-establishment procedure followed in an LTE network will be described in the following. It will be appreciated that other networks may have similar, but not necessarily identical, re-establishment procedures.
A network node, such as eNB, controlling a target cell receives a recovery request message from the UE, such as an RRC connection re-establishment request. In response to this message, the target eNB may respond with an RRC connection re-establishment message sent to the UE, by which the target eNB accepts the re-establishment request. The message may include various configuration parameters, so that the connection can be properly adapted and continued in the new cell. Other message names may of course apply, such as any reference to cell re-selection or handover.
Upon reception of the re-establishment message, the UE may now process the content of that message and resume the RRC connection according to the content and commands provided therein. Typically, the UE further sends a confirm message to the eNB of the target cell, where the confirm-message indicates that the communication between the UE and the target eNB can now resume. For example, the RRC connection re-establishment request, the re-establishment message, and subsequent confirm-message may typically include fields for supporting secure identification of the UE and fields for supporting contention resolution, i.e., such that the UE and its connection can be unambiguously and securely identified.
The recent and rapid uptake of mobile broadband data services has led to a need for increasing the capacity of cellular networks. One solution to achieve such a capacity increase is to use denser networks consisting of several layers of cells with different sizes: macro cells ensure large coverage with cells encompassing large areas, while micro-, pico- and even femto-cells are deployed in hot-spot areas where there is a large demand for capacity. Those cells typically provide connectivity in a much smaller area, but by adding additional cells and radio base-stations controlling those cells, capacity is increased as the new cells off-load the macros. Such networks are referred to as heterogeneous networks. FIG. 3 shows a UE 700 moving from the coverage of a pico-cell A into the coverage of macro cell B.
The different “layers” of cells can be deployed on the same carrier, i.e., in a reuse-1 fashion. Alternatively, small cells can be deployed on one or more different carriers, and the different cells on the various layers can even be deployed using different technologies, e.g., 3H/HSPA on the macro- and micro-layer, and LTE on the pico-layer as one non-exclusive example.
There is currently a large interest in investigating the potential of such heterogeneous networks. However, it has also been found that heterogeneous networks may result in an increased rate of handover failures and other radio-link failures. One reason is that the handover region in heterogeneous networks may be very small, meaning that the handover might fail since the user equipment loses coverage from the source cell before a handover to a target cell can be completed. For example, when a user equipment leaves a pico-cell, it may happen that the coverage border of the pico is so sharp, that the user equipment fails to receive any handover command towards a macro before losing coverage to the pico.
FIG. 4 illustrates handover regions for a macro-to-pico handover and a macro-to-macro handover respectively; it can be seen that the former is significantly smaller than the latter. As a result, it may not be “seen” at all by a mobile terminal that is moving quickly through the region.
Similar problems can occur when a UE connected to a macro cell suddenly enters a pico cell on the same carrier. It can happen that the control channels of the pico interfere with the signals that the UE needs to receive from the macro in order to complete the handover, for example, and the handover thus fails.